1. Field of the Invention
The present invention generally relates to a multicylinder four-cycle combustion engine for use primarily in motorcycles and, more particularly, to the multicylinder four-cycle combustion engine of a kind in which the piston pumping loss can be reduced.
2. Description of the Prior Art
In order to reduce the number of component parts of the multicylinder four-cycle combustion engine and, also, to reduce the number of manufacturing steps, it is well known that some of multicylinder four-cycle combustion engines currently used in motorcycles are of a structure in which a cylinder block and a generally upper half of a crankcase are formed integrally with each other. In the case of such multicylinder four-cycle combustion engine, crank chambers one for each cylinder are separated from each other by means of partition walls. Therefore, in order to alleviate the piston pumping loss which would result from as a result of compression of an air within each of the crank chambers that takes place as the corresponding piston moves, the design has been employed, in which the crank chambers for the neighboring cylinders are communicated with each other by means of a communication hole open at one end with an upper or lower portion of one crank chamber and at the other end with an upper or lower portion of the other crank chamber. Each of those communication holes has a round section and is formed by the use of a drilling technique, in which a generally elongated drill is inserted from a position laterally of an engine casing in a direction parallel to the longitudinal axis of the crankshaft, so as to extend transversely between the neighboring crank chambers. See, for example, the Japanese Laid-open Patent Publication No. 11-182325.
In the known multicylinder four-cycle combustion engines of the structure discussed above, since each communication hole is formed by the use of a drilling technique, it has been found that burrs tend to be formed around the leading end of the respective communication hole with respect to the direction of advance of the drill, through which the tip of the drill emerges outwardly. In order to prevent the piston ring from interfering with the burrs so formed and appearing in the inner peripheral wall of the corresponding cylinder bore, the position of each of the communication holes has necessarily and carefully be chosen so that the uppermost edge of the respective communication hole with respect to the direction of movement of the associated piston be located 3 mm or more spaced downwardly from the lowermost end of the piston ring when the piston is held in the bottom dead center position.
Thus, the position of the uppermost edge of the respective communication hole is necessarily limited to a location distant from the lowermost end of the associated piston ring when the piston is held in the bottom dead center position and, on the other hand, the lowermost edge of the respective communication hole must be positioned at a location sufficient to avoid interference with a crank shaft bearing. Those design requirements impose limitations on the size of the leading open end of the respective communication hole, particularly the size of the leading open end as measured in a direction conforming to the direction of reciprocating movement of the piston or a vertical direction.
Since each of the communication holes has a round section as discussed above, the size of the open end in the vertical direction for a given cross-sectional surface area (passage area) of the respective communication hole tends to be large, and accordingly, it is difficult to secure a sufficient passage area for the communication hole under the limitations on such vertical size. Moreover, the presence of the burrs around the open end of the communication hole tends to impose a relatively large resistance to the flow of gases through the communication hole.
Partly because of the insufficient passage area for each communication hole and partly because of the relatively large resistance to the gas flow caused by the burrs, the gases would not flow smoothly therethrough, resulting in increase of the pumping loss. Also, openings through which the drill has been inserted to form the respective communication holes, are left in the lateral portions of the engine casing and, therefore, those openings must be closed by separately prepared plugs, resulting in increase of the number of component parts used and, also, the member of assembling steps.